Increasing the circuit voltage of a power source used for arc welding greatly improves the welding performance and arc stability of the process. This is especially true for AC welding operations where output circuit is commanded to switch between positive and negative polarity. In this situation, it is important to reestablish the arc immediately upon polarity reversal, both positive and negative to positive, in order to maintain arc stability in the AC welding process whether it is AC TIG or AC MIG. This is also true in AC submerged arc welding. As described in prior patents, the background technology for AC welding often involves an output stage having a center tapped choke. The purpose of this choke is well known and operates well under most applications. The choke arrangement utilizes the stored energy in the core of the choke to maintain current flow in the same direction in both sections of the center tapped choke irrespective of the actual welding polarity. In theory, the center tapped choke develops whatever voltage is required to maintain the current flow in either the positive or negative direction. The limitation of this design is the amount of stored energy available to reignite the welding arc at the moment of polarity reversal. The stored energy is proportional to the square of the current through the sections of the center tapped choke multiplied by ½ the inductance of the choke. In most AC welding applications, this energy is more than adequate to reignite or reestablish the welding arc when there is a change in polarity. However, there are conditions where there is not enough energy to consistently reignite the arc; therefore, the center tapped choke must be quite large to accomplish more energy storage. Larger chokes are more costly and they also impede the welding performance of AC welding. In some instances, when the choke is on the common leg of the output circuit, energy must be dissipated during each polarity cycle of the AC welding process. In this situation, there is not enough energy to reestablish consistently the arc at polarity reversals. Thus, there is a need for an output stage or circuit to assure sufficient open circuit voltage to reignite the arc in opposite directions during polarity reversal and AC welding process without merely increasing the capacity of the power source during the output circuit.
The main welding output of a standard power source used for electric arc welding (this phrase includes plasma arc cutting) usually develops an open circuit voltage of less than about 80 volts. The typical arc voltage is usually less than 30 volts. Thus, at reversal of polarity, there is only about 50 volts open circuit voltage to reestablish the arc. In addition to this 50 volts would be the voltage produced by the output choke. This total voltage, however, is sometimes insufficient to reestablish the welding arc. This is especially true at low current welding operations, such as welding at less than 10 amperes as is common in AC TIG welding. Low open circuit voltage for the power source creates high efficiency; however, the power source has difficulty maintaining the welding arc especially at longer arc lengths. For instance, in short arc welding, a low open circuit voltage is generally not enough to reignite consistently the arc at polarity reversals. Consequently, the output voltage for a power source, especially for AC MIG welding, must be high enough to maintain the arc during times of long arc lengths. Furthermore, higher voltage output from the power source inverter reduces the efficiency of the inverter. However, there is a need for a higher open circuit voltage, especially at polarity reversal in AC welding process. A solution would be to increase the open circuit voltage of the main output circuit of the power source. This is expensive and drastically reduces the efficiency of the power source. Consequently, the need for a high open circuit voltage for a standard AC welding presents a dilemma. Furthermore, a high open circuit voltage should not be available at the output terminals of the inverter used as the power source when the inverter is not driving a welding operation. There is a need for a circuit to provide high open circuit voltage for an AC welding process when high open circuit voltage for the power source itself is not sufficient. These needs are solved efficiently by the present invention relating to a novel output stage or output circuit for the power source of an electric arc welder capable of AC welding.